1. Field of the Invention
The invention relates to a process for the preparation of saturated alcohols by hydrogenating aldehydes in the gas phase in several stages using an alkaline copper catalyst in a first stage and a nickel-containing catalyst in a second stage.
2. Discussion of the Background
It is known that alcohols can be prepared by catalytic hydrogenation of the corresponding saturated and unsaturated aldehydes.
Thus, DE-A-21 50 975 describes the preparation and use of a highly active nickel catalyst on an SiO.sub.2 carrier, which contains less than 0.2% of sodium and is also suitable for the hydrogenation of aldehydes. For example, 2-ethylhexanal is hydrogenated using a low hydrogen/aldehyde molar ratio of only 2.5:1 at 36 bar in the liquid phase. The conversion is 96%.
In hydrogenations in the liquid phase, plant pressures of 20 to 300 bar ar usually employed in order to achieve sufficiently complete hydrogenation. Since the reaction is highly exothermic, recycling of a considerable part of the hydrogenated product or dilution with a solvent for capacitive heat removal is necessary in industrial reactors. This permits only relatively low throughputs of aldehydes through the reactors.
These difficulties are avoided by hydrogenation in the gas phase. Thus, in DE-C-1 152 393, gas-phase hydrogenation is carried out in two stages. A copper catalyst is used in the first stage and a copper/nickel catalyst in the second stage. In the gas-phase hydrogenation according to DE-C-1 161 250, a copper catalyst is used in the first stage and a nickel catalyst in the second stage. In both patents, kieselguhr (diatomaceous earth) or kieselguhr modified with sodium phosphate is used as the carrier. The surface of these carriers is neutral or slightly alkaline.
DE-B-1 227 882 describes a two-stage gas-phase hydrogenation over a copper and a palladium catalyst, and a copper/nickel catalyst may also be used in between. Only neutral or slightly alkaline carriers are used here. The copper and nickel contents of the supported catalysts are very high. Furthermore, the products still have considerable residual aldehyde contents.
DE-C-1 276 620 describes a one-stage hydrogenation in the gas-phase over a copper/nickel supported catalyst. However, the comparative example advises against a two-stage hydrogenation over a copper catalyst and then over a copper/nickel catalyst.
According to DE-C-1 276 618, the gas-phase hydrogenation can be carried out in two stages. Here, a copper/nickel catalyst is used in the first stage and a nickel or palladium catalyst in the second stage.
A further improvement in the gas-phase hydrogenation is described in DE-C-1 643 856. Here, the hydrogenation is carried out over silica gel supported catalysts containing copper and/or nickel. The pH of the surface of the silica gel is adjusted to 6-10. The "surface pH" is determined, inter alia, by the method of O. Johnson, J. Phys. Chem. 59, 827 (1955). For aldehydes Which are difficult to hydrogenate and in the case of high space velocity, conventional supported nickel and/or palladium catalysts may also be used downstream. Although no information is given about the surface pH of the downstream catalysts, in this case too the patent suggests a pH of 6 to 10, especially since comparative examples make a distinction with respect to catalysts whose surface pH is less than 6 or greater than 10.
We have found that in this case the formation of hydrocarbons increases sharply and the selectivity decreases substantially at high space velocity.
In DE-A-37 37 277, the hydrogenation, which preferably takes place in two pressure stages, is carried out over a copper/zinc oxide catalyst doped with an alkali metal and/or a transition metal. The catalyst is preferably impregnated with potassium and nickel. According to this patent application, the hydrogenation is improved by the use of an alkaline catalyst.